Generally, electronics control an automatic transmission system of a vehicle through precisely controlling various valve actuators. Certain of these valves control the hydraulic pressure associated with the planetary gear sets, thereby providing precise shift control for stationary and moving vehicles. In other words, various engine sensors transmit signals, relating to operating conditions of the vehicle, to a transmission control unit ("TCU"). Based on these operating conditions, the TCU controls both shift mode hydraulic pressure and a damper clutch of the automatic transmission to optimize the performance of the vehicle.
The TCU controls various solenoid valve actuators to control the automatic transmission pressure. In other words, the TCU generates a control signal for each solenoid valve. The control signal corresponds to an open/shut position of the solenoid valve. The position of the valves control the hydraulic pressure within the hydraulic control system, which improves shift quality of the transmission system.
To achieve the above described control, detection of the vehicle's running condition, by each input sensor, should be accurate, and TCU should be programmed precisely. By this method, the automatic transmission maintains the hydraulic pressure to achieve the optimal running state of the vehicle under all road and operating conditions. Maintaining this condition requires a minimal amount to no effort on the part of the driver. By adding a control logic, which does not exist in a manual transmission, the automatic transmission is able to accomplish this optimal running state.
The transmission control system, of the prior art, generally has 6 shift modes (or ranges), e.g., a parking "P" range, a reverse "R" range, a neutral "N" range, a drive "D" range, a second "2" range, and a low "L" range. Additionally, the "D" range provides first through fourth forward speed ratios, the "2" range the first and second forward speed ratios, and the "L" range the first forward speed ratio. That is, the driver can select one of the shift modes by shifting a shift selector lever between the modes, "D", "2", or "L". A shift operation occurs when the vehicle is placed in a mode other then "P" or "N".
The TCU controls the forward and reverse speed ratios of the "R", "D", "2", and "L" ranges by controlling the hydraulic control system. The TCU generates the control signal necessary to control the hydraulic pressure based on various engine sensors, including engine rpms and throttle position. In particular, the vehicle's running condition is detected by various sensors, and the detected signals are transmitted to the TCU, such that each speed ratio in one of the "D", "2", and "L" ranges is determined in accordance with a shift pattern programmed in the TCU. When the automatic transmission vehicle is in a forward driving state, the vehicles's running conditions that the TCU uses to control hydraulic pressure include both engine rpms from a vehicle speed sensor ("PG-B"), and a throttle opening sensor's voltage from a throttle position sensor ("TPS"). The TCU then determines the speed ratio in response to the rmp and throttle position signals.
In the conventional automatic transmission, as described above, the transmission control system is designed to drive forwardly when the shift selector lever is in the "D" range. However, the signal supplied to the TCU does not have a direction of motion detector. Therefore, it is possible for the vehicle to inadvertently reverse while the shift selector lever is in the "D" range. For example, a vehicle stopped on a gradient may roll backwards with the shift selector still in a "D" range. The TCU would measure the backwards roll and assume that the vehicle was in forward motion. This phenomenon is generally known as creeping. That is, in this state, the inadvertent reversing of the vehicle cannot be prevented by the TCU as long as the driver does not depress the brake. The conventional automatic transmission, therefore, has a limitation of being unable to control creeping in the "D" range.
This is caused because the conventional automatic transmission is constructed such that the creep control programmed in the TCU can prevent inadvertent reversing on a road having a gentle gradient but cannot prevent the same on a road having a steep gradient. In other words, when the throttle position and engine rpm indicate a stopped state, the TCU applies pressure to the kick down band which prevents the automatic transmission planetary gears from turning. This pressure is capable of preventing the gears from turning on flat as well as gentle gradient. When the road is steeply sloped upward, relatively higher hydraulic pressure should engage respective clutches and brakes to prevent the vehicle from reversing. However, there is no control logic which can achieve this in the conventional automatic transmission.
Additionally, the conventional TCU sensor is designed to generate a frequency signal proportional to the vehicle speed so as to identify the current vehicle speed. However, the conventional TCU design does not discriminate the change between forward and reverse states of the vehicle. That is, when the vehicle is reversed in a state where the shift selector lever is in the drive "D" range, the TCU takes the vehicle's reverse speed for the forward speed and controls the hydraulic control system according to this misidentification.
The creep control of the conventional automatic transmission and the conventional vehicle speed sensor will be described hereinafter with reference to FIGS. 1, 2A and 2B.
As shown in FIG. 1, the conventional method of creep control includes, at step s1, calculating the rotating speed of a transfer drive gear and, at step s10, calculating the rotation speed of a vehicle speed reed switch. These signals constitute the frequency signal. Next, step 12, the method determines if the shift operation is performed or not. As described above, a shift operation exists when the vehicle shift selector lever is in a position other then "P" or "N". At step s14, the method transmits a signal relating to the frequency signal generated in steps s1 and s10 to a shift control part, in the cases where the shift operation is performed. Step s16, the TCU determines whether creep is occurring, based on industry standard methods. Step s16 is only performed when the shift operation is not performed. Step s18, transmitting the signal to a control routine, such that normal control occurs when creeping does not occur; and transmitting the signal to a creep control routine when creeping occurs, step s20.
As described above, the conventional creep control method has a drawback in that it does not consider the rotating direction of the transfer drive gear, that is, the driving direction of the vehicle. Thus, the TCU can inadvertently control hydraulic pressure based upon rolling backwards.
The inadvertent control signal is generated because the conventional TCU has no ability to sense the direction of motion of a vehicle. Referring to FIGS. 2A and 2B, there are shown a structure of a conventional vehicle speed reed switch (speed sensor) and an output wavy pattern, produced by the speed reed switch, induced into the TCU, respectively, illustrating a structural problem of the sensor. The symmetrical design of vehicle speed reed switch, as shown in FIG. 2A, produces a wavy pattern, used as the vehicle speed signal, induced into the TCU, incapable of indicating vehicle direction. In other words, the wavy pattern is the same regardless of the rotating direction of the transfer drive gear, as shown in FIG. 2B. The vehicle speed is measured by the rotational speed of the transfer drive gear. As FIGS. 2A and 2B show, the TCU cannot discriminate vehicle direction, i.e., between forward and reverse driving states, based on the wavy pattern produced by the vehicle speed reed switch.
More in detail, when the system determines the following 4 conditions are satisfied, then the system determines creep is occurring and that creep control is necessary. In the prior art, when it is determined creeping is occurring, the creep control achieves the second speed and generates a pressure control solenoid valve duty ratio of 68.8%. The 4 conditions indicating creep are:
1) The manual selecting signal should be in the "D" or "2" range;
2) The rotation speed of the transfer drive gear is lower than 460 rpm;
3) The idle switch is ON (i.e., the accelerating pedal should not be pressed); and
4) The throttle opening voltage is lower than 0.94 V.
FIG. 3 shows a pressure control solenoid valve duty output during the creep control. As shown in FIG. 3, when the rotation speed of the transfer drive gear is lower than 460 rpm, the creep duty ratio is maintained constantly at 68.8%.
Further, when satisfying one of the following 4 conditions, the vehicle exits the creep state:
1) The manual selecting signal relays being in the P, R, N, or L range;
2) The rotation speed of the transfer drive gear is higher than 460 rpm;
3) The idle switch is OFF, (i.e., the accelerating pedal should not be pressed); and
4) The throttle opening voltage should be higher than 0.94 V.
As described above, the conventional creep control method in the "D" range of the automatic transmission is performed without consideration of the rotating direction relayed by the vehicle speed sensor. It is desirous to identify the vehicle direction to more accurately control the hydraulic press.